In the field of automobiles, ships or many other mechanical constructions such as aircrafts, mechanical components such as shafts, gears and cams are often required to be of certain wear resistance and surface toughness. To obtain such resistance and toughness, methods such as carburization and/or nitration may be employed for the surface treatment that may be involved.
For instance, China publication CN203360578U discloses a sample carburization method where a protective cover is used to provide certain shielding for the carburization procedure.
For instance also, U.S. publication U.S. Pat. No. 2,398,809 discloses a process of case-hardening hollow metal members where an opening is sealed by glaze-coated ceramic devices.